1. Field
The present invention relates to modulation spectroscopy. More specifically, it relates to a tunable system to be used as a narrowband modulation spectrometer operating at Terahertz frequencies.
2. Related Prior Art
Systems are known, which make use of FM modulation spectroscopy (FMS). These FMS systems involve devices in the visible or near-IR spectral regions. Typically, a single laser source is FM modulated, either directly or via an external modulator, to produce the desired sidebands, centered about the optical carrier. However, in order to realize more versatile FMS systems or communications or RADAR systems, it is necessary to modulate an optical or RF source over a wide excursion at a high frequency. The required frequency excursion and modulation frequency (e.g., modulation index or number of sidebands) are dictated by the spectroscopic feature to be detected in the case of FMS or by the system bandwidth in the case of a communications link. Although such systems exist in the optical and infrared regimes as well as in the RF domain (GHz range), they have not been realized in the THz regime.
Modulation of a THz carrier cannot be realized via direct modulation of a THz source, since such high-frequency modulators, i.e. having a frequency modulation Δ in the THz range, are not available, especially in the case of high-modulation index applications, where an external modulator would require many waves of phase shift at high frequencies, or, an oscillator would require a large fractional bandwidth, in the case of direct frequency modulation of a THz oscillator.
U.S. Pat. No. 4,733,397 to Gallagher discloses a resonant cavity optical modulator, and U.S. Pat. No. 4,765,736 to Gallagher discloses a frequency modulation spectroscopy technique using dual frequency modulation and detection. However, in both documents the modulation is imposed onto a single laser beam. Therefore, should FMS in the THz regime be performed by means of the techniques disclosed in Gallagher, it could only be performed by a single THz source, whose carrier frequency is that of the THz oscillator itself. Further, the Gallagher documents do not disclose how to generate all the required modulation sidebands required for the FMS applications, like, for example, remote sensing.
In order to operate at Terahertz frequencies, two kinds of approaches have been followed.
According to a first approach, a pulsed, broadband Terahertz probe is employed, to detect amplitude and/or phase shifts of a sample in a given volume. The broadband Terahertz radiation is generated using short-pulsed lasers, incident either onto a photoconductive switch or onto a nonlinear optical difference-frequency mixer. The detection portion employs either a matching photoconductive switch or an electro-optic crystal. The system is broadband, with about 10% to 50% bandwidth, because of the use of pulsed lasers, which contain many frequency components. As a consequence, the system has limitations in terms of sensitivity, selectivity and resolution.
According to a second approach, narrowband measurements of the amplitude and/or phase shift experienced by a probe beam in the presence of the given species are effected. A fixed-frequency narrowband source is employed (e.g. a free-running narrowband oscillator, or direct modulation of a laser using a high-frequency stable modulator) or alternatively a tunable narrowband source. An example of a narrowband tunable source involves a pair of laser sources, whose frequency difference corresponds to the desired Terahertz frequency, and is incident upon a fast detector (e.g. group III-V FETs or HEMTs—high electron mobility transistors), with the output coupled in a microwave antenna. This system is narrowband and can be tuned by tuning the lasers. However, the pair of lasers are tuned to a fixed frequency difference. This can generate a THz output signal, which contains only a single non-tunable frequency component corresponding to the frequency difference of the two lasers. As a consequence, it is not useful for high-resolution spectroscopy, since, in many cases, the desired spectral feature to be sensed is obscured by other spurious absorption features (like broadband absorption and scattering), which are not able to be compensated by a system using a single carrier frequency.
As a consequence, there is a need for a system that generates a multi-frequency THz output beam to enable ultra-high-precision spectroscopy and trace-compound detection.